1. Field of the Invention
The present invention relates to a method for pressurized mud cap and reverse circulation drilling from a floating structure using a sealed marine riser while drilling. In particular, the present invention relates to a method for pressurized mud cap and reverse circulation drilling from a floating structure while drilling in the floor of an ocean using a rotating control head.
2. Description of the Related Art
Marine risers extending from a wellhead fixed on the floor of an ocean have been used to circulate drilling fluid back to a floating structure or rig. The riser must be large enough in internal diameter to accommodate the largest bit and pipe that will be used in drilling a borehole into the floor of the ocean. Conventional risers now have internal diameters of approximately 20 inches, though other diameters are and can be used.
An example of a marine riser and some of the associated drilling components, such as shown in FIG. 1, is proposed in U.S. Pat. No. 4,626,135, assigned on its face to Hydril Company, which is incorporated herein by reference for all purposes. Since the riser R is fixedly connected between the floating structure or rig S and the wellhead W, as proposed in the '135 patent, a conventional slip or telescopic joint SJ, comprising an outer barrel OB and an inner barrel IB with a pressure seal therebetween, is used to compensate for the relative vertical movement or heave between the floating rig and the fixed riser. Diverters D have been connected between the top inner barrel IB of the slip joint SJ and the floating structure or rig S to control gas accumulations in the subsea riser R or low pressure formation gas from venting to the rig floor F.
One proposed diverter system is the TYPE KFDS diverter system, previously available from Hughes Offshore, a division of Hughes Tool Company, for use with a floating rig. The KFDS system's support housing SH, shown in FIG. 1A, is proposed to be permanently attached to the vertical rotary beams B between two levels of the rig and to have a full opening to the rotary table RT on the level above the support housing SH. A conventional rotary table on a floating drilling rig is approximately 49½ inches in diameter. The entire riser, including an integral choke line CL and kill line KL, are proposed to be run-through the KFDS support housing. The support housing SH is proposed to provide a landing seat and lockdown for a diverter D, such as a REGAN diverter also supplied by Hughes Offshore. The diverter D includes a rigid diverter lines DL extending radially outwardly from the side of the diverter housing to communicate drilling fluid or mud from the riser R to a choke manifold CM, shale shaker SS or other drilling fluid receiving device. Above the diverter D is the rigid flowline RF, shown configured to communicate with the mud pit MP in FIG. 1, the rigid flowline RF has been configured to discharge into the shale shakers SS or other desired fluid receiving devices. If the drilling fluid is open to atmospheric pressure at the bell-nipple in the rig floor F, the desired drilling fluid receiving device must be limited by an equal height or level on the structure S or, if desired, pumped by a pump up to a higher level. While the choke manifold CM, separator MB, shale shaker SS and mud pits MP are shown schematically in FIG. 1, if a bell-nipple is at the rig floor F level and the mud return system is under minimal operating pressure, these fluid receiving devices may have to be located at a level below the rig floor F for proper operation. Hughes Offshore has also provided a ball joint BJ between the diverter D and the riser R to compensate for other relative movement (horizontal and rotational) or pitch and roll of the floating structure S and the fixed riser R.
Because both the slip joint and the ball joint require the use of sliding pressure seals, these joints need to be monitored for proper seal pressure and wear. If the joints need replacement, significant rig downtime can be expected. In addition, the seal pressure rating for these joints may be exceeded by emerging and existing drilling techniques that require surface pressure in the riser mud return system, such as in underbalanced operations comprising drilling, completions and workovers, gas-liquid mud systems and pressurized mud handling systems. Both the open bell-nipple and seals in the slip and ball joints create environmental issues of potential leaks of fluid.
Returning to FIG. 1, the conventional flexible choke line CL has been configured to communicate with a choke manifold CM. The drilling fluid then can flow from the manifold CM to a mud-gas buster or separator MB and a flare line (not shown). The drilling fluid can then be discharged to a shale shaker SS to mud pits and pumps MP. In addition to a choke line CL and kill line KL, a booster line BL can be used. An example of some of the flexible conduits now being used with floating rigs are cement lines, vibrator lines, choke and kill lines, test lines, rotary lines and acid lines.
The following patents and published patent applications, assigned to assignee of the present invention, Weatherford/Lamb, Inc., propose floating rig systems and methods, and are incorporated herein by reference in their entirety for all purposes: U.S. Pat. No. 6,263,982, entitled “Method and system for return of drilling fluid from a sealed marine riser to a floating drilling rig while drilling”; U.S. Pat. No. 6,470,975, entitled “Internal riser rotating control head”; U.S. Pat. No. 6,138,774, entitled “Method and apparatus for drilling a borehole into a subsea abnormal pore pressure environment”; U.S. Patent Application Publication No. 20030106712, entitled “Internal riser rotating control head”; and U.S. Patent Application Publication No. 20010040052, entitled “Method and system for return of drilling fluid from a sealed marine riser to a floating drilling rig while drilling.”
The '982 patent proposes a floating rig mud return system that replaces the use of the conventional slip and ball joints, diverter and bell-nipple with a seal below the rig floor between the riser and rotating tubular. More particularly, the '982 patent proposes to have a seal housing, that is independent of the floating rig or structure for receiving the rotatable tubular, with a flexible conduit or flowline from the seal housing to the floating structure to compensate for resulting relative movement of the structure and the seal housing. Furthermore, the '982 patent proposes the seal between the riser and the rotating tubular would be accessible for ease in inspection, maintenance and for quick change-out.
In addition, it has been known in onshore drilling to use a mud cap for increasing bottomhole pressure. A mud cap, which is a column of heavy and often viscosified mud in the annulus of the well, has a column shorter than the total vertical depth (TVD) of the annulus. A mud cap can typically be used to control bottomhole pressure on a trip and to keep gas or liquid from coming to the surface in a well, resulting in total lost circulation. The size of the mud cap is based on, among other factors, how long the cap needs to be, the mud weight of the cap, and the amount of extra pressure that is needed to balance or control the well.
When a single pass drilling fluid is used, the mud cap can also prohibit fluid and cuttings from returning from downhole. Rather, the mud cap in the annulus directs mud and cuttings into a zone of high porosity lost circulation, sometimes known as a theft zone. While a theft zone, when drilling conventionally, can cause undesirable excessive or total lost circulation, differentially stuck pipe, and resulting well control issues, mud cap drilling takes advantage of the presence of a theft zone. Because the theft zone is of high porosity, relatively depleted, and above the production zone, the theft zone offers an ideal depository for clear, non-evasive fluids and cuttings. In one mud cap drilling technique, pressurized mud cap drilling (PMCD), well bore pressure management is achieved by pump rates. One further requirement of a mud cap concerns the resistance of the mud to contamination in the well bore, its viscosity, and its resistance to being broken up by flow or circulation, which depend on the purpose of the mud cap, the size of the hole, the mud in the hole, and the formation fluid. Mud from a mud cap used on a trip is generally stored and reused on the next trip.
FIG. 13 is an elevational view showing a prior art onshore well 1300 using mud cap drilling. A mud cap 1330 is placed in the annulus 1350 surrounding the drill pipe 1320, capping the return flow from the borehole 1360 upwards through the annulus 1350. Cuttings and debris are shown extending outward from the borehole into a lost circulation area 1340. This mud cap drilling technique is well known for onshore wells and offshore fixed wells, but has been unavailable for offshore floating rigs because of the inability to handle the vertical and horizontal movements of the floating rig structure relative to the annulus, while sealing the top of the riser.
Although PMCD has been used in onshore drilling, PMCD has been unavailable for use offshore on floating rigs, such as semi-submersible rigs. The ability to use PMCD offshore on floating rigs would be desirable.